The present invention relates to a process for manufacturing lids, in particular lids for beverage cans. Such lids are most often made of aluminum (pure aluminium or an aluminium alloy), tin plate or tin free steel (TFS) and are usually coated, at least on the inside face, with a protective layer of lacquer e.g. a phenolic epoxy based lacquer.
It is known to provide such lids with a pouring hole or group of holes and, if desired, at least one air inlet hole.
These holes are then covered over with a sealed on, pull-off closure strip. Such strips are, advantageously, made of an approximately 0.9-0.14 mm thick aluminium foil which is coated on the side to be sealed to the lid with a coating of thermoplastic plastic e.g. about 0.015-0.03 mm thick, usually about 0.02 mm thick. Before applying that plastic layer, however, the aluminium is usually coated first with a thinner layer of phenolic epoxy lacquer. The outside of the strip is usually covered with a decorative lacquer coating. Strips of aluminum-polyamide laminate, in particular with a polyamide-12 coating, have proved to be very suitable for this purpose.
For production reasons the lid material is lacquered in strip or sheet form and the lids prepared from that with the pouring and, if desired, airing holes being punched out of the material in the process. The result is, however, that the cut edges around the holes are then left unprotected. These unprotected edges are undesireable as they are exposed to corrosive attack by the contents, in particular by such ones containing CO.sub.2 ; the corrosion products could lead to spoiling of the contents. Such a phenomenon is already found to occur to a moderate degree with aluminium lids; it is however more pronounced in the case of lids made of ferrous material (tin plate or TFS). For example at a concentration of only 1 mg Fe-ions per liter beer becomes cloudy, also some cola drinks at a concentration of 1.5 mg Fe per liter.